Method of removing acid formed during cathodic electrodip coatings

ABSTRACT

A method of removing, by oxidation at the anode, the acid liberated in cathodic dip-coating in the course of the deposition of the coating film, using anodes coated with a layer of ruthenium oxide, iridium oxide or tin oxide or with a mixture of these oxides.

[0001] The present invention relates to a method of removing the acidliberated from the electrodeposition bath in cathodic electrodepositioncoating while the coating film is being deposited.

[0002] In cathodic electrodeposition coating, the substrate to be coatedis immersed in an aqueous electrodeposition bath and connected as thecathode. When a voltage is applied, a coating film is deposited on thesubstrate. The coating materials employed comprise polymers that havebeen converted by protonation to a water-dispersible form. Thisprotonation is achieved predominantly through the addition of weakorganic acids. These acids accumulate in the region of the anode if inthe course of coating deposition the cationic binder is neutralized andthe coating material that has been consumed is gradually replaced bynew, protonated coating material. In order to control the pH of theelectrodeposition bath, therefore, it is necessary to remove the acidfrom the bath. This is generally done by means of what is known as theanolyte circuit. The anolyte circuit initially requires that the anodeis separated from the remainder of the electro-deposition bath by adiaphragm or membrane. This membrane is generally an anion exchangemembrane, which permits only anions—in the present case, acidradicals—to flow toward the anode. Binders and pigments, on the otherhand, are held back. By means of the anolyte circuit, acid-enrichedelectrolyte is withdrawn from the anode compartment, discarded generallyas wastewater, and replaced by water.

[0003] In addition to the anolyte circuit, electrodeposition bathsnormally include an ultrafiltration circuit. This circuit removes bathliquid directly and passes it to an ultrafiltration stage whose purposeis to separate out solvents and other coating components of lowmolecular mass that accumulate in the bath. Binders and pigments, on theother hand, are retained and passed back to the bath. For furtherdetails regarding the prior art reference may be made to the literature(e.g. “farbe+lack”, February 1981, p. 94 ff).

[0004] For removing the acid which accumulates in the bath U.S. Pat. No.3,682,814 proposes breaking down at least part of this acid by oxidationat the anode. Auxiliary measures are considered here for effectiveimplementation of the oxidation, such as the heating of the anode zoneor the addition of a catalyst solution. It is additionally intended thatanodes be used which are coated with platinum, platinum oxide and othernoble metals, chromates, manganates, vanadates, molybdates, cobalt,nickel, chromium and oxides of these metals, and other heavy metals. Theacid to which U.S. Pat. No. 3,682,814 gives particular preference in itsprocedure is formic acid. The reason for this is that the oxidativedissociation of formic acid to carbon dioxide and water consumes twocharge units per dissociated molecule. The acid in the bath cantherefore be broken down electrochemically with a theoretical maximum of50%. Other common electrocoating bath acids require more charge unitsper molecule for their anodic oxidation to carbon dioxide and water, andtherefore have even lower theoretical maximums.

[0005] Using the process specified in U.S. Pat. No. 3,682,814 it ispossible according to Example 1 therein to break down about 40%—out of atheoretical maximum possible 50%—of the formic acid neutralized at theanode. A disadvantage of this process is the instability of the anodesemployed. In addition, anode and cathode are separated by a membrane,since the temperatures and pH for a particularly efficient reaction inthe anode compartment are different than those prevailing in the cathodecompartment.

[0006] DE-44 09 270 also proposes anodic oxidation of the acid employed.As already preferred in U.S. Pat. No. 3,682,814 the acid employed hereis again essentially—in other words, to an extent of more than90%—formic acid. Theoretically possible anode materials specified areplatinum and platinized stainless steel electrodes, platinized titaniumelectrodes, platinized graphite electrodes, ruthenium-doped stainlesssteel electrodes or mixed-oxide-doped electrodes made from stainlesssteel, titanium or graphite. Unlike U.S. Pat. No. 3,682,814, however,this patent gives no measurement results and no numerical data for thebreakdown rates achieved.

[0007] With regard to the electrodes that can be employed inelectrochemical processes, German Patent 15 71 721 discloses electrodesthat are employed inter alia as anodes for the chloralkali electrolysis.To avoid losses and improve electrode resistance, use is made of oxidesof platinum, iridium, rhodium, palladium, ruthenium, manganese, lead,chromium, cobalt, iron, titanium, tantalum, zirconium or of silicon.Further fields of use of said electrodes are in electrodialysis andelectrodeposition coating.

[0008] German Patent 16 71 422, furthermore, discloses the use in thealkali metal chloride electrolysis of an anode comprising a titaniumcore with a mixed coating covering at least part of the core surface andcomprising a material formed from ruthenium oxide and titanium oxidewhich is resistant to the electrolytes and to the electrolysis products.These anodes exhibit a substantially lower degree of overvoltage and atthe same time are dimensionally stable. Building on these properties itwas possible to develop cell constructions, such as membrane cells, andhence to improve the performance of the mercury cells and diaphragmcells known hitherto.

[0009] DE-A 34 23 605 describes composite electrodes comprising anelectroconductive polymer and, embedded partly therein, catalyticparticles (support particles with applied catalyst) and processes fortheir preparation. They can be employed, for example, as an oxygen anodein the electrolytic recovery of metals from aqueous solutions. Furtherfields of use that are specified are electro-dialysis andelectrodeposition coating.

[0010] EP-B 0 296 167 likewise describes the use of comparableelectrodes (referred to as dimensionally stable anodes, DSA) in cathodicelectrodeposition coating. These electrodes are neither dissolved nordestroyed in the course of electrophoretic coating under theelectrodeposition conditions assumed therein, i.e. in respect of coatingformulation, current density, pH and the destructive influence ofchlorine.

[0011] In the field of wastewater treatment, as well, various electrodeshave been tested and employed for the oxidative breakdown of substances.In the course of such tests and use it has been found, in particular,that electrodes with a coating of tin oxide (SnO₂) are highly effectivein the electrochemical breakdown of organic substances. These electrodeshave in particular also been tested in comparison with conventionalelectrodes such as the abovementioned DSA electrodes, for example(Stucki, Kötz, Carcer, Suter: “Electrochemical waste water treatmentusing high overvoltage anodes, Part II: Ahode performance andapplications”, Journal of Applied Electrochemistry 21 (1991), 99-104;Comninellis: “Traitement des eaux résiduaires par voie électrochimique”,gwa November 1992, 792-797; Comninellis: “Electrochemical treatment ofwaste water containing phenol”, Electrochemical Engineering and theEnvironment 92, Symposium Series No. 127, 189-201).

[0012] The present invention has now set itself the object of providinga method of removing the acid liberated in cathodic electrodepositioncoating in the course of the deposition of the coating film whichreduces the number of rinsing procedures via the anolyte circuit thatare required to remove acid and possible breakdown products of the acidor which manages completely without the anolyte circuit.

[0013] This object has surprisingly been achieved by removing theliberated acid, preferably formic acid, by oxidation at anodes coatedwith a layer of ruthenium oxide, iridium oxide or tin oxide or with amixture of these oxides. With the procedure of the invention it has beenpossible to break down, by oxidation, more than 49% of the acid in thebath. This comes close to the theoretical maximum of 50% and is aconsiderable improvement on the levels of around 40% specified in U.S.Pat. No. 3,682,814. Such increased efficiency makes it possible toreduce the number of flushing procedures required to remove the acid viathe anolyte circuit. Furthermore, the electrodes employed by theinvention are more chemically stable toward the medium.

[0014] It is also possible in accordance with the invention to employacids other than formic acid. Such acids, however, are generally lessfavorable owing to their lower theoretical maximum capacity forelectrochemical breakdown. Lactic acid, for instance, can be broken downelectro-chemically only to an extent of about 35%.

[0015] Surprisingly, it is even possible to do entirely without theanolyte circuit; in other words, the electrodes of the invention can beemployed directly in the electro-deposition bath and it is no longernecessary to separate the anode from the cathode compartment by amembrane. In this case, the invention employs additional methods toreduce the acid content. These methods preferably comprise conventionalmembrane methods. These methods preferably begin with the ultrafiltrate,since the latter is already devoid of the relatively high molecular massconstituents of the coating material. Examples of suitable membranemethods are methods operating by means of dialysis, osmosis, reverseosmosis, electrodialysis or a further downstream ultrafiltration.Methods of this kind are described, for example, in DE-44 09 270, EP-262419, U.S. Pat. No. 4,971,672 or U.S. Pat. No. 5,091,071.

[0016] As demonstrated by Example 2 below, it is possible under theseconditions—i.e. without the anolyte circuit—to remove about 65-70% ofthe acid from the bath. The remaining 30-35% of acid can be expelled bymeans of a membrane method, e.g. electrodialysis. The use of suchadditional measures is of course also possible in baths operating withan anolyte circuit.

[0017] In the method of the invention the electrodeposition coatingbaths preferably employed are those whose binders comprise syntheticresins that have cationic groups. These binders are preferablyprotonated reaction products of epoxide-functional synthetic resins andamines. Protonation here preferably involves formic acid, acetic acid,lactic acid and dimethylolpropionic acid. Special preference is given tothe use of formic acid. These acids are oxidized at the anode into waterand carbon dioxide.

[0018] The substrate of the electrodes of the invention can consist ofmetal or conductive plastic. In the case of the metals it preferablycomprises titanium, tantalum, niobium or an alloy of these metals. Asuitable example is an alloy of titanium and from 1 to 15% by weightmolybdenum. A particularly preferred substrate is titanium.

[0019] The layer of ruthenium, iridium or tin oxide, or of a mixture ofthese oxides, that is applied to the anodes employed in accordance withthe invention preferably has a thickness of from 0.01 to 10 μm. Aparticularly preferred range is 0.1-7 μm.

[0020] The text below describes experimental examples with the method ofthe invention, demonstrating especially the improvements over the priorart.

EXAMPLE 1

[0021] Efficiency of the Anodic Breakdown of Formic Acid as a Functionof the Electrode Material

[0022] In a rectangular vessel (L×W×H=20×10×20 cm) fitted with astainless steel cathode, 2.5 l of an aqueous formic acid solution(approximately 0.2 mol/l) were oxidized with different anode materials.The current density was 5 mA/cm², with an electrolyte temperature of25-30° C. After each 0.2 F/mol step (based on formic acid), the acidcontent was determined by potentiometry. After 1 F/mol the experimentwas terminated, and the acid break-down and current yield, orefficiency, of the anodic oxidation of acid were determined.

[0023] Acid breakdown=(c₀−c_(t))/c₀

[0024] c_(o)=concentration of acid prior to experiment

[0025] c_(t)=concentration of acid at time t of sampling or after theend of the experiment

[0026] In determining the current yield (efficiency), the theoreticallyrequired charge consumption of 2 F/mol of acid is taken into account forthe conversion of the formic acid to CO₂:

[0027] Efficiency=(c₀−c_(t))/c₀*Q₀/Q_(t)

[0028] Q₀=theoretical charge consumption, i.e. 2 F/mol

[0029] Q_(t)=actual charge consumption

[0030] The anodes employed (each 10×10 cm) were as follows:

[0031] a. stainless steel (1.4401)

[0032] b. RuO₂-coated titanium plate

[0033] C. IrO₂-coated titanium plate

[0034] d. SnO₂-coated titanium plate

[0035] The table which follows shows the results of the measurements.From this table it is evident that the anodes of the invention achievean efficiency of from 93.7 to 98.8%, as compared with 86.9% forconventional stainless steel electrodes.

[0036] With values from 48.2 to 49.2%, the amount of acid broken downelectrochemically almost reaches the theoretical maximum of 50%. Tablefor Example 1 Electrode type Stainless steel (1.4401) RuO₂ IrO₂ SnO₂Charge Acid Acid Acid Acid consump- Acid break- Effic- Acid break-Effic- Acid break- Effic- Acid break- Effic- tion content down iencycontent down iency content down iency content down iency [F/ mol](mol/l) % % (mol/l) % % (mol/l) % % (mol/l) % % 0.0 0.198 0.199 0.1990.198 0.2 0.181  8.6 85.9 0.178 10.6 105.5 0.181  9.0  90.5 0.181  8.685.9 0.4 0.165 16.7 83.3 0.16 19.6  98.0 0.16 19.6  98.0 0.161 18.7 93.40.6 0.146 26.3 87.5 0.141 29.1  97.2 0.139 30.2 100.5 0.14 29.3 97.6 0.80.127 35.9 89.6 0.122 38.7  96.7 0.118 40.7 101.8 0.123 37.9 94.7 1.00.111 43.9 87.9 0.103 48.2  96.5 0.101 49.2  98.5 0.102 48.5 97.0Average efficiency 86.9  98.8  97.8 93.7

EXAMPLE 2

[0037] Experiment on Acid Breakdown in a SemiautomaticElectro-deposition Coating Unit (Plate Coater)

[0038] Electrodeposition Coating Material:

[0039] A. Binder Dispersion (cf. EP 074 634, Example C, but Neutralizedwith Formic Acid)

[0040] The example that follows shows the preparation of a cationicresin that is neutralized by formic acid. Bisphenol A, bisphenol Adiglycidyl ether and a bisphenol A/ethylene oxide adduct are heatedtogether and form a modified polyepoxy resin. A blocked iso-cyanate isadded as crosslinker to this resin. The product is then reacted with amixture of secondary amines. The resin is partly neutralized with formicacid and is dispersed in water. Starting materials Parts by weightEpikote 828¹ 682.44 Bisphenol A 198.36 Dianol 265² 252.70 Methylisobutyl ketone 59.66 Beuzyldimethylamine 3.67 Blocked isocyanate³1011.28 Diketimine⁴ 65.41 Methylethanolamine 59.65 1-Phenoxy-2-propanol64.77 Formic acid 85% 32.92 Emulsifier mixture⁵ 15.217 Demineralizedwater 3026.63

[0041]¹Liquid epoxy resin prepared by reacting bisphenol A andepichlorohydrin, having an epoxide equivalent weight of 188(manufacturer: Shell Chemicals)

[0042]²Ethoxylated bisphenol A having an OH number of 222 (manufacturer:Akzo)

[0043]³Polyurethane crosslinker prepared from diphenylmethanediisocyanate, where of 6 mols of isocyanate 4.3 are reacted first withbutyldiglycol and the remaining 1.7 mol with trimethylolpropane. Thecrosslinker is in the form of an 80% strength solution in methylisobutyl ketone and isobutanol (9:1 by weight).

[0044]⁴Diketimine formed from the reaction of diethylenetri-amine andmethyl isobutyl ketone, 75% strength in methyl isobutyl ketone

[0045]⁵Mixture of 1 part of butyl glycol and 1 part of a tertiaryacetylene glycol (Surfynol 104, manufacturer: Air Products)

[0046] Epikote 828, bisphenol A and Dianol 265 are heated to 130° C. ina reactor with nitrogen blanketing. Then 1.6 parts of thebenzyldimethylamine (catalyst) are added, the reaction mixture is heatedto 150° C. and maintained at between 150 and 190° C. for about half anhour, and then cooled to 140° C. Subsequently, the remainingbenzyldimethylamine is added and the temperature is held at 140° C.until, after about 2.5 h, an epoxide equivalent weight of 1120 isestablished. Directly thereafter, the polyurethane crosslinker is addedand the temperature is lowered to 100° C. The mixture of the secondaryamines is added subsequently, and the reaction is maintained at 115° C.for about 1 h until a viscosity of about 6 dPas is reached (50% dilutionin methoxypropanol, ICI cone and plate viscometer). Following theaddition of phenoxypropanol the resin is dispersed in the water in whichthe formic acid and emulsifier mixture have been dissolved.

[0047] The solids content after this step is 35%, and rises to 37% afterthe low-boiling solvents have been stripped off. The dispersion ischaracterized by a particle size of about 150 nm.

[0048] B. Grinding Resin (cf. EP 505 445, Example: Grinding Resin A3)

[0049] A reactor equipped with stirrer, internal thermometer, nitrogeninlet and water separator with reflux condenser is charged with 30.29parts of an epoxy resin based on bisphenol A and having an epoxideequivalent weight (EEW) of 188, and with 9.18 parts of bisphenol A, 7.04parts of dodecylphenol and 2.37 parts of butyl glycol. This initialcharge is heated to 110° C., 1.7 parts of xylene are added, and thexylene is distilled off again under a weak vacuum together with anypossible traces of water. Then 0.07 part of triphenylphosphine are addedand the mixture is heated to 130° C. After an exothermic heat rise to150° C., reaction is continued at 130° C. for 1 h. The EEW of thereaction mixture is then 860. The mixture is then cooled, during which9.91 parts of butyl glycol and 17.88 parts of a propylene glycoldiglycidyl ether with an EEW of 333 (DER 732, Dow Chemical) are added.At 90° C., 4.23 parts of 2-(2′-anilinoethoxy)ethanol and, 10 minuteslater, 1.37 parts of N,N-dimethyl-aminopropylamine are added. After ashort period of exothermicity the reaction mixture is held at 90° C. for2 h more until the viscosity remains constant, and is then diluted with17.66 parts of butyl glycol. The resin has a solids content of 69.8%(measured at 130° C. for 1 h) and a viscosity of 5.5 dPas (40% strengthin Solvenon PM; cone and plate viscometer at 23° C.). The base contentis 0.88 meq/g of solid resin (meq=milliequivalent=mmol of acid or base).

[0050] C. Pigment Paste (cf. EP 505 445, Example: Pigment Paste B 3, butNeutralized with Formic Acid)

[0051] To prepare the pigment paste a premix was first formed from 34.34parts of deionized water, 0.38 part of formic acid (85% strength) and18.5 parts of grinding resin. Then 0.5 part of carbon black, 6.75 partsof extender (ASP 200), 37.28 parts of titanium dioxide (R 900) and 2.25parts of crosslinking catalyst (DBTO) are added and the constituents aremixed for 30 minutes in a high-speed dissolver stirrer. The mixture isthen dispersed to a Hegman fineness of less than 12 for 1 to 1.5 h in alaboratory ball mill and adjusted with further water, if necessary, tothe desired processing viscosity.

[0052] D. Electrodeposition Coating Material

[0053] For the cathodic electrodeposition coating material, 36.81 partsof the binder dispersion A are diluted with 52.5 parts of deionizedwater, and 10.69 parts of pigment paste C are introduced into thismixture with is stirring. The coating material has a solids content ofabout 20% with an ash content of 25%.

[0054] The plate coater, equipped with pump circulation, temperatureregulation unit, an attached ultrafiltration unit, but without separateanolyte circuit, is filled with 8 l of the above-describedelectrodeposition coating material.

[0055] The anode used is a titanium electrode (measuring 10×10 cm),coated with iridium oxide, which is immersed directly into theelectrodeposition coating material. In the coater, steel panels(measuring about 10×20 cm) are coated automatically for 2 minutes at 280V and at 28° C. The coat thickness is about 20 μm. After coating, thepanels are dipped in the ultrafiltrate in order to rinse off adheringcoating material and thereby pass it back to the dip tank.

[0056] After each 50 coated panels the CED material is analyzed and isreplenished with binder and pigment paste. The meg acid analyses showthe change in the acid content as a function of the replenishment rate.FIG. 1 shows a plot of the meq acid content against the “turnover” ofthe CED bath (a turnover of 1 denotes complete replenishment of thebath). Also indicated is the change in acid content at 0 and 100% acidexpulsion.

[0057] Acid expulsion 0% =doubling of the meq acid value after 1turnover

[0058] Acid expulsion 100=constant meg acid value FIG. 2 shows the acidexpulsion based on the meq acid value after 0.3 turnover (the 0.3turnover base was chosen since at this point in time the establishmentof equilibrium between bath and ultrafiltrate is virtually complete).

Acid expulsion=(dturnover×meqSo+meqSo−meqSt)/dturnover×meqSo

[0059] where:

[0060] dturnover=actual turnover=0.3

[0061] meqSo=meq acid content after 0.3 turnover (base value, see above)

[0062] meqSt=actual meq acid content

[0063] Summary: The breakdown rate or expulsion rate, after relativelyhigh levels to start with (establishment of equilibrium), settles downto a constant level of 65-70% over the period of the experiment. Thebalance of 100% acid expulsion must be achieved by means of anadditional measure, such as electrodialysis, for example.

1. A method of removing the acid liberated in cathodic electrodepositioncoating in the course of the deposition of the coating film, whichcomprises breaking down the acid by oxidation at an anode which iscoated with a layer of ruthenium oxide, iridium oxide or tin oxide orwith a mixture of these oxides.
 2. The method as claimed in claim 1,wherein said oxidation takes place in an anolyte circuit.
 3. The processas claimed in claim 1, wherein anode and cathode are not separated fromone another by a membrane in the electrodeposition bath and wherein inaddition to the anodic oxidation a further separation of acid takesplace with the aid of a membrane method.
 4. The method as claimed inclaim 3, wherein an ultra-filtration circuit is present in theelectrodeposition coating bath and the membrane method is carried outwith the ultrafiltrate.
 5. The method as claimed in either of claims 3and 4, wherein the membrane method is a dialysis, electro-dialysis,osmosis, reverse osmosis or a second ultrafiltration.
 6. The method asclaimed in one of claims 1 to 5, wherein the anodes are employed inelectrodeposition coating baths whose binders comprise synthetic resinsthat have cationic groups.
 7. The method as claimed in one of claims 1to 6, wherein the synthetic resins are protonated reaction products ofepoxide-functional synthetic resins and amines.
 8. The method as claimedin one of claims 1 to 7, wherein formic acid, acetic acid, lactic acidand dimethylol-propionic acid are oxidized at the anode.
 9. The methodas claimed in one of claims 1 to 8, wherein anodes are employed whosesubstrate comprises metal or conductive plastic, the metal usedpreferably being titanium, tantalum, niobium or an alloy of these metalssuch as titanium with 1-15% by weight molyb-denum, for example.
 10. Theuse of an anode coated with a layer of ruthenium oxide, iridium oxide ortin oxide or with a mixture of these oxides to remove, by means ofoxidation, the acid liberated in cathodic electrodeposition coating inthe course of the deposition of the coating film.
 11. The use of theanode as claimed in claim 10 to oxidize formic acid, acetic acid, lacticacid and dimethylolpropionic acid.